This invention relates to the treatment of hydrocarbon oils and, more particularly, to the hydrocracking of heavy hydrocarbon oils to produce improved products of lower boiling range.
Hydrocracking processes for the conversion of heavy hydrocarbon oils to light and intermediate naphthas of good quality for reforming feed stocks, fuel oil and gas oil are well known. These heavy hydrocarbon oils can be such materials as petroleum crude oil, atmospheric tar bottoms products, vacuum tar bottom products, heavy cycle oils, shale oils, coal-derived liquids, crude oil residuum, topped crude oils and heavy bituminous oils extracted from tar sands. Of particular interest are oils extracted from tar sands and which contain wide boiling range materials from naphthas through kerosene, gas oil, pitch, etc. and which contain a large portion of material boiling above 524.degree. C. These heavy hydrocarbon oils contain nitrogen and sulphur compounds in extremely large quantities and often contain excessive quantities of organo-metallic contaminants which tend to be detrimental to various catalytic processes which may subsequently be carried out, such as hydrofining. Of the metallic contaminants, those containing nickel and vanadium are most common, although other metals are often present. These metallic contaminants, as well as others, are usually present within the bituminous material as organo-metallic compounds of relatively high molecular weight. A considerable quantity of the organo-metallic complexes are linked with asphaltenic material and contain sulphur.
As the reserves of conventional crude oils decline, the heavy oils must be upgraded to meet the demands. In this upgrading, the heavier material is converted to lighter fractions and most of the sulphur, nitrogen and metals must be removed. This is usually done by means of a hydrocracking process.
In catalytic hydrocracking, the mineral matter present in the feed stock tends to deposit on the surface of the expensive catalyst, making it extremely difficult to regenerate, again resulting in increased production cost. The non-catalytic or thermal hydrocracking process can give a distillate yield of over 85 weight percent but in this process, there is a very considerable problem of the formation of coke deposits on the wall of the reactor which ultimately plug the reactor and cause costly shutdowns.
It is known to recycle downstream heavy hydrocarbon products in thermal hydrocracking processes for the purpose of improving efficiency. For instance, Wolk, U.S. Pat. No. 3,844,937, issued Oct. 29, 1974 describes a process for utilizing a high ash content in the hydrocracking zone fluid e.g. in the range of 4-10 weight percent as a means for preventing the formation of coke in the hydrocracking zone. In order to achieve this ash content in the fluid, a recycle of heavy hydrocarbons from a hot separator was used and as a part of this recycle, the heavy hydrocarbons from the hot separator were passed through a cyclone or through another low pressure separator. This was carried out at quite low recycle rates and, consequently, quite low liquid up-flow velocities in the hydrocracking zone.
Another prior system utilizing recycle of separator bottoms is Schlinger et al. U.S. Pat. No. 3,224,959, issued Dec. 21, 1965. In that procedure, the heavy hydrocarbons from the hot separator were contacted with a separate hydrogen stream heated to a temperature between 800.degree. and 950.degree. F. and this hydrogen treated product was then recycled into the hydrocracking zone. This procedure involved extremely high hydrogen recirculation rates of up to 95,000 s.c.f/b.b.l. making the procedure very expensive. Moreover, the reaction zone was operated at a high turbulence which resulted in reduced pitch conversion with high operating and production costs.
In Ranganathan et al, U.S. Pat. No. 4,435,280, issued Mar. 6, 1984, a process is described in which a feed slurry of heavy hydrocarbon oil and coal particles was passed upwardly through a vertical hydrocracking zone while a drag stream of liquid content of the hydrocracking zone was drawn off. A portion of this drag stream could be recycled to the feed slurry. However, there are no examples showing that the recycle was ever used and it cannot be seen that there would be any particular benefit in doing so. Thus, the recycle would only remove liquid from the hydrocracking and feed it back in where it came from.
Another patent which describes recycle is Khulbe et al, U.S. Pat. No. 4,252,634. This describes a process for hydrocracking heavy hydrocarbon oils with recycle of heavy oil from a downstream hot separator. The purpose of this recycle was to increase the superficial liquid upflow velocity in the hydrocracking zone to at least 0.25 cm/sec such that deposition of coke in the hydrocracking zone was substantially eliminated. Mixed effluent from the top of the hydrocracking zone was also discharged into the hot separator vessel in a lower region below the liquid level to provide vigorous mixing action in the bottom of the separator, thereby also substantially preventing coke deposits in the hot separator.
In Unger et al, U.S. Pat. No. 4,411,768, issued Oct. 25, 1984 a recycle is used in a catalytic hydrogenation operation with a fractionated heavy product stream being recycle to a hydrogenation stage. This process is carried out in an ebullated catalytic bed and there is no catalyst in the recycle.
It is an object of the present invention to provide a process for hydrocracking heavy hydrocarbon oils in which additive particles are included in the feedstock to suppress coke formation and downstream fractionated heavy product is recycled to the feedstock with active additive particles being retained in the recycle.